To what extent was Elizabeth regarded as Mary's probable successor before Mary's own succession?
As of 1553, Mary was a 37-year-old spinster. I'd think that many thought she would not have children.
Of course this not really parallel to James II's accession in 1685; James was deposed despite his pledge to respect the Protestant status of England, while Mary acceded in spite of her clear intent to suppress Protestantism.
Certainly Elizabeth was regarded as Mary's rightful successor as of 1553. That was one of the stumbling blocks to Northumberland's attempt to place Jane on the throne; Jane was not next in line. If Elizabeth is not present, do Northumberland and Edward alter the succession sooner and more authoritatively?
Also, without Elizabeth, where does Jane actually stand? If she is already heir presumptive to the heir presumptive, her marriage to Guildford Dudley becomes unlikely - a future Queen regnant would be reserved for a proper royal match.
Hey, there's a TL idea: Elizabeth dies in 1551 of sweating sickness. Northumberland tries to marry Jane to Guildford, but it blows up in his face and he is removed as Lord Protector. (Who replaces him?) Mary succeeds, marries Philip, and dies childless. Does Jane now succeed uncontested? Mary would prefer a Catholic successor over Jane, but OTL she would have preferred a Catholic successor over Elizabeth.
Here's yet another random idea: WI Mary doesn't develop uterine cancer (but remains childless), and lives to be say 68? That gives her an additional 25 years to suppress Protestantism in England. It also blocks both of Philip's later marriages, and he probably dies without offspring. That shakes up the Spanish succession.